Direct XAFS evidence for heterogeneous redox reaction at the aqueous chromium/magnetite interface

Hexavalent chromium is a highly toxic, carcinogenic, and mobile contaminant present in wastewaters from mining and industrial operations. Its reduction to trivalent chromium, both less toxic and less soluble over the pH range of most natural waters, has previously been observed in solutions in contact with the redox-sensitive iron oxide magnetite (Fe2+Fe23+O4), and occurs via electron transfer from Fe2+ in the magnetite structure. This study presents direct in situ X-ray absorption fine structure (XAFS) evidence for the presence of Cr(III), initially resulting from the reduction of Cr(VI)aq in solution, at the surface of synthetic magnetite at near-neutral pH. Cr(VI) reacts with freshly-synthesized magnetite at a surface coverage of 4.5 μmol m−2 to be entirely reduced to Cr(III), as evidenced by the Cr absorption edge position and by the absence of a 1s → 3d pre-edge peak. XAFS spectra of Cr on progressively oxidized magnetite surfaces, however, show increasing pre-edge peak height indicating the presence of Cr(VI), i.e. a decrease in the Cr-reducing capacity of altered (maghemite-coated) magnetite grains. As expected, Cr(VI) sorbed to a ferric oxide, synthetic maghemite (γ-Fe23+O3), is not significantly reduced. The Cr pre-edge peak height for Cr(VI) reacted with maghemite is comparable to pre-edge peak heights of Cr(VI) model compounds. XAFS spectra for Cr model compounds compare well with theoretical XAFS spectra for the same compounds calculated using the ab initio, single- and multiple-scattering code FEFF. Fit parameters derived from FEFF models of Cr(III)- and Cr(VI)-containing compounds were used to determine the coordination environment (number and chemical identity of neighboring shells of atoms and their interatomic distances) of Cr sorbed to magnetite and maghemite samples.